The brazing of aluminum involves joining surfaces of aluminum or aluminum alloy components with a brazing alloy, i.e. an alloy of aluminum with a melting point substantially lower than that of the components. The components are joined by juxtaposing them with the brazing alloy adjacent to or between them, and heating to a temperature that will effect melting of the brazing alloy without melting the components.
A flux consisting of salts of alkali metals or alkaline earth metal chlorides or fluorides is employed to remove aluminum oxide coatings present on the exposed metal and brazing alloy surfaces and to promote the flow of brazing alloy while brazing aluminum or aluminum alloys. Fluoride-based fluxes such as mixtures of K.sub.3 AlF.sub.6 and KAlF.sub.4, classified as non-corrosive fluxes, are generally preferred over chloride-based fluxes for brazing aluminum or aluminum alloys because they (1) are nonhygroscopic and substantially water insoluble after brazing, (2) leave water insoluble residue after brazing, and (3) are inert or non-corrosive with respect to aluminum or aluminum alloys at brazing temperatures.
Aluminum or aluminum alloy components are generally brazed using the above fluoride-based (or non-corrosive) fluxes in the presence of a protective inert atmosphere such as pure nitrogen gas, as described in U.S. Pat. Nos. 3,951,328, 3,971,501, 4,475,960, and SAE Technical Paper 780300 titled "Furnace Brazing Aluminum with a Non-Corrosive Flux" published in 1978, and SAE Technical Paper 830021 titled "Brazing Aluminum Automotive Heat Exchanger Assemblies Using a Non-Corrosive Flux Process" published in 1983. Some of these fluoride-based, non-corrosive fluxes are marketed by the Aluminum Co. of Canada, Ltd. under the tradmark NOCOLOK. The use of an inert, pure nitrogen gas facilitates brazing of aluminum or aluminum alloys with good brazed joint quality consistently and reproducibly while utilizing low loading of a non-corrosive flux.
Japanese Patent Application No. 63-205318 discloses a method for brazing aluminum or aluminum alloys using a non-corrosive flux in a modified continuous furnace. According to this application, pure nitrogen gas is made to flow in such a way that the inside of the heating zone of the furnace has a nitrogen atmosphere with a dew point of -40.degree. C. or below and an oxygen concentration of 1,000 ppm or less. The thrust of the invention is to minimize air infiltration into the furnace.
Inert, pure nitrogen gas is produced by cryogenic distillation of air. It contains very low moisture and residual oxygen (less than -60 C. dew point and 5 ppm of residual oxygen). Since inert, pure nitrogen gas produced by cryogenic distillation of air is expensive to produce, it is increasingly becoming uneconomical to continue using use it for brazing aluminum or aluminum alloys.
Aluminum or aluminum alloys can be brazed in an inexpensive room air provided high loading (large quantities) of a non-corrosive flux is employed. High loading of a flux is used primarily to compensate for the presence of high levels of oxygen and moisture in the room air. Besides being expensive, the use of high loading of a flux is not desirable because it leaves a large amount of flux residue on the parts resulting in an unattractive appearance of the brazed assembly.
In view of the foregoing state of the art a need existed for a low-cost atmosphere process for brazing aluminum or aluminum alloys with consistent good brazed joint quality and appearance while utilizing low loading of a non-corrosive flux.